The present invention relates to a mobile tamping machine for tamping ballast under a main track, which comprises a machine frame supported on undercarriages running on the main track for moving the machine frame in an operating direction, the main track defining a track plane, a ballast tamping unit mounted on the machine frame between the undercarriages, a main track lifting and lining unit vertically and transversely adjustably mounted on the machine frame immediately ahead of the ballast tamping unit in the operating direction, drive means for vertically and transversely adjusting the main track lifting and lining unit, an auxiliary lifting unit mounted on the machine frame for lifting a branch track branching off the main track at a track switch, and a measuring system for controlling lifting of the track switch.
U. S. Pat. No. 4,905,604 discloses a machine of this type. According to the patent, a special reference system is required to control the lifting of the branch track. The references system comprises a measuring axle running on the main track and a measuring axle running of the branch track. The two measuring axles are coupled together by a rod carrying a cross level. This enables an operator remote-controlling the auxiliary branch track lifting unit to conform the lifting of the branch track accurately to that of the main track by monitoring the cross level.
It is the primary object of this invention to provide a tamping machine of the indicated type which enables an exact conformity between the lifting of the main and branch tracks at a switch to be effected in a simple manner.
This and other objects are accomplished in a machine of the first-described structure according to the invention by providing a common measuring system including an emitter of a light beam forming a reference line extending parallel to the track plane, the light beam emitter being positioned at a fixed distance from the main track, and a receiver of the light beam recording the reference line, the light beam receiver being positioned at a fixed distance from the branch track.
The fixed distance may be zero, if desired, i.e. the light beam emitter and receiver may contact the main and branch tracks, respectively.
Such a common measuring system automatically and accurately coordinates the lifting of the main and branch tracks at the switch. In this way, an accurate and swift track position correction can be effected without any problems even in track switches which have long ties with elastic joints.